1. FIELD OF THE INVENTION
The present invention relates to a method for bending a glass plate for a laminated glass.
2. DISCUSSION OF BACKGROUND
When a laminated glass having a curved surface is to be manufactured, it is necessary for a plurality of glass plates , (for instance, two glass plates) to be laminated to have the same radius of curvature and shape. Otherwise, there occur such problems as that the glass plate may be broken during bending operations, or undesired bubbles and a white-cloudy phenomenon may be result in a mating surface of the laminated glass when it is in use, or the mating surfaced may be releaced in use.
As a method of bending two glass plates used for manufacturing the laminated glass having a curved surface, there has been proposed a deadweight bending method in which the two glass plates are placed in an overlapping state on a bending ring mold having a predetermined shaping surface to coincide the radius of curvature and the shape of the glass plates, followed by heating them, whereby the two glass plates are simultaneously bent along the curved surface of the bending ring mold by its deadweight at a temperature which causes the softening of the glass plates. However, when the deadweight bending method is used in the case that a print-coating layer such as colored ceramics paint or an electric conductive paste is previously provided on the mating surface of the two glass plates, there occurs mutual close-contact between the glass plates through the print-coating layer when the bending operation is carried out at a high temperature. Therefore, in the deadweight bending method, an additional process is required. Namely, the print-coating layer has to be formed by previously baking at such a temperature that the glass plates do not deform themselves in an oven, after the print-coating layer has been dried.
On the other hand, there have been proposed a pressing method and a vacuum suction method for bending the glass plates for the laminated glass other than the above-mentioned deadweight bending method. The former is, for instance, to place each of the glass plates on each of bending ring molds or to hang each of the glass plates, followed by bending it by using a pair of pressing molds, and the latter is to place at least two glass plates one by one on a mold having a predetermined shaping surface followed by subjecting each of the glass plates to suction for the bending operation.
Even in the above-mentioned conventional methods, it was still possible to form glass plates having good contacting properties so as to render the radius of curvature and the shape of the glass plates to be in coincidence with each other when they are bonded in the case that the laminated glass to be manufactured does not have a very complicated curved surface.
In recent years, there have increasingly been demands for a laminated glass having a complicated curved surface such as one having a three-dimensional curved surface which is formed by projecting the glass plates in a steric form, one having a curved surface in substantially S-like form in cross-section difficult to form it, and one having both side parts formed by the deep-bending of them.
When the laminated glass having such complicated curved surface is manufactured, it is necessary to further increase the degree of softness in at least two glass plates to be bent. Accordingly, it is necessary to conduct a shaping process by elevating the temperature at the surface of the glass plates to the extent of 630.degree. C. or higher.
However, when the conventional deadweight bending method is carried out under the condition of such a high temperature, a parting composition such as sodium bicarbonate or zeolite which is previously provided on the mating surfaces between the glass plates to avoid the close-contact of them, fine particles of curret, or glass powder produced in a cutting process or in a chamferring process may protrude into or deposit on the glass plates, with the result that undesirable optical strain may be produced. Further, it is difficult for the deadweight bending method to impart such complicated curved surfaces to the glass plates.
On the other hand, in the above-mentioned pressing method or vacuum suction method, it is difficult to obtain the same radius of curvature and the shape for the glass plates to be laminated because they have to be bent one by one.
Accordingly, there still remain such technical problems that coincidence in shape of the glass plates and surface fitness of the glass plates can not be obtained in the manufacture of the laminated glass having the complicated curved surface.